Improved tube for a heat exchanger

ABSTRACT

A heat exchanger element includes a tubular body with a wall at least partly delimited by an inner surface and an outer surface. The wall has a twisted shape on a segment of the body. The inner surface has a groove with a shape corresponding to the wall. The groove extends helically over the segment. On the segment, the outer surface has a diameter between 18 and 30 millimeters, the groove has a pitch of less than 3.5 millimeters and a depth such that the ratio of the pitch at a real power between 1.5 and 2.5 to the depth is less than a threshold value close to 24. A heat exchanger, for example of condenser type, can include such an element.

The invention relates to an element for a heat exchanger of industrialtype, in particular a condenser, of the type comprising a generallytubular body.

Condensers comprising such elements, also known as “tube condensers” arewidely used in industry, in particular for electricity production. Afirst fluid, typically water in the liquid state, is circulated inside aplurality of tubes, while a second fluid in the gaseous state, generallysteam, is brought into proximity with the tubes, outside them. Heat isthen exchanged between the first and second fluids, through the wall ofthe tubes, which exchange causes the fluid in the gaseous state tocondense.

Condensers of the industrial type must be able to condense largequantities of steam in as short a time as possible. The volume of steamthat they are capable of condensing per unit of time at least partlycharacterises their performance. To this end, condensers of theindustrial type are generally fitted with hundreds, or even thousands,of great-length tubes, typically up to around twenty metres long.

Originally, condensers of the industrial type were fitted with smoothtubes. To improve their performance, in particular regarding the flowrate of condensed steam, tubes of a new type started to be used, thebody of which still retains its generally tubular shape, but the wall ofwhich has a twisted shape extending over at least one segment of saidbody. This twisted shape of the wall results in an outer surface with adomed relief extending helically along the segment in question, and acorresponding groove on the inner surface of the body.

This particular configuration significantly improves the heat exchangesat the tubes: on the one hand, the twisted shape gives the wall a largercontact area between the fluids, both on the inside and the outside ofthe tubes; on the other hand, it causes turbulence in the fluid flowinginside the tubes, which is beneficial overall for the heat exchanges atthe tubes. The twisted shape also improves the drainage of the dropsthat form on the outer surface of the tubes.

In the art, tubes with this type of configuration are known as“corrugated” or, more correctly, “equipped with corrugations”. The pitchof the twist, also known as the corrugation pitch, is generally greaterthan 20 millimetres.

The applicant has noted that, in general, the actual performance of thecorrugated tubes, in particular regarding their ability to condense afluid on their outer surface, is quite significantly lower than theexpected performance.

The invention aims to improve the existing situation.

The proposed heat exchanger element comprises a tubular body the wall ofwhich is at least partly delimited by an inner surface and an outersurface. The wall has a twisted shape on at least one segment of saidbody. The inner surface has at least one groove with a shapecorresponding to said wall that extends helically over said segment. Onsaid segment, the outer surface has a diameter comprised between 18 and30 millimetres, while the groove has a pitch of less than 3.5millimetres and a depth such that the ratio of the pitch at a real powercomprised between 1.5 and 2.5 to the depth is less than a thresholdvalue close to 24.

The invention will be better understood on reading the followingdetailed description, with reference to the attached drawings, in which:

FIG. 1 shows a diagram of a generic heat exchanger;

FIG. 2 is a top view of a tube element for the exchanger in FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of a wall portion of atube element for the exchanger in FIG. 1;

FIG. 4 shows a partially cut away perspective view of a twisted portionof a tube element for a heat exchanger.

The attached drawings contain elements of certain character. They maytherefore be used not only to supplement the description of theinvention, but also contribute to the definition thereof, as applicable.

Reference is made to FIG. 1. It shows, generically, a heat exchanger ofindustrial type in the form of a condenser 1.

The condenser 1 comprises a plurality of individual tubes 3 held inrelation to each other in one or more bundles 5 by plates 7 distributedalong the tubes 3. Each plate 7 is thus passed through by each of thetubes 3 in the bundle 5.

The condenser 1 also comprises a pair of header tanks 9 into which theopposite ends of each of the tubes 3 respectively open out.

One of the tanks 9 is in fluid communication with a fluid inlet 11,while the other tank 9 is in fluid communication with a fluid outlet 13.

The inlet 11 and outlet 13 can be connected to the rest of a circuit inwhich a first fluid circulates. Typically, the first fluid enters thecondenser 1 through the inlet 11 in liquid form. It circulates from thecorresponding header tank 9 to the other header tank inside the tubes 3in one or more passes. From there, the first fluid leaves the condenser1 for the rest of the circuit through the outlet 13.

The bundle 5 of tubes 3 is housed in an enclosure 15 provided insidewhat is known in the art as a shell 17. The shell 17 is equipped with afluid inlet 19 and a fluid outlet 21 that open out into the enclosure15.

The inlet 19 and outlet 21 allow the condenser 1 to be connected to acircuit in which a second fluid circulates.

The second fluid enters the enclosure 15 through the inlet 19 in gaseousform. On contact with the tubes 3, the second fluid exchanges heat withthe first fluid circulating inside these tubes. As the first fluid isgenerally introduced at a lower temperature than that of the secondfluid, the latter condenses on the outer surface of the tubes 3. Thesecond fluid in liquid form leaves the enclosure 15 through the outlet21.

Condensers of the type of the condenser 1 are widely used in industrialelectricity production. In particular, steam is condensed by means ofcold water circulating inside the tubes. To this end, great-length tubeseach up to around twenty metres long are used.

Reference is made to FIG. 2. It shows a tube element TE that can be usedin a condenser of the type of the condenser 1.

The tube element TE comprises a hollow elongated generally cylindrical,or tubular, body BDY with a length TL. The body BDY has two longitudinalend sections ES1 and ES2 connected to each other by a central section CSwith a length CL. The length TL corresponds to the total length of thetubular element TE, including the central section CS and the endsections ES1 and ES2.

The end sections ES1 and ES2 are generally cylindrical, with an outsidediameter TOD. The diameter TOD, as it is usually known in the art,corresponds to the nominal outside diameter of the element TE. The endsections ES1 and ES2 each have a smooth outer and inner surface.

The central section CS has a wall that extends along the body in atwist, or helically, or in a helix, forming spirals LP around thelongitudinal axis LA of the tube element TE. Here, the spirals LP arecontiguous.

This twisted configuration of the wall of the element TE results in anouter surface that, over the length of the central section CS, has ahelical relief, made of hollows and bosses. This relief is likely toimprove the heat exchange capabilities of the element TE, because theouter surface thereof is then more extensive than that of a smooth tubewith the same outside diameter. Furthermore, it improves the drainage ofthe drops that form on the outer surface of the element TE. The centralsection CS retains the general appearance of a hollow cylinder, havingan outside diameter COD.

Reference is made to FIGS. 3 and 4. These show, generically, a twistedportion CW of the wall and a configuration of said twisted shape. Thetwisted shape of the wall CW results in an inner surface IS with arelief made of peaks and hollows, with a shape correspondingrespectively to the hollows and bosses on the outer surface OS. In otherwords, the inner surface IS has a groove that extends along a helix withcontiguous spirals along the central section CS. In yet other words, theinner surface IS has a helical shape.

This relief is likely to improve the heat exchange capabilities of theelement TE, due to the generation of eddies in the fluid flowing insidethe element TE.

The twisted wall CW has a thickness TT. The thickness TT corresponds tothe nominal thickness of the element TE, i.e. the thickness of the wallof the smooth tube on which the element TE is based. The central sectionCS has an inside diameter CID. The diameter CID corresponds to thediameter of a gauge just capable of passing through the inside of theelement TE.

The twisted section CS has an outside diameter COD that corresponds tothe nominal outside diameter of the element TE on the twisted sectionCS, i.e. the diameter of a cylindrical envelope surface of said section.

The twisted shape has a pitch CP, if applicable considered inside thetubular element. The depth CD of the inner groove resulting from thetwisted shape is considered in relation to an inner envelope surface ofthe tubular element or, in other words, as the radial distance betweenthe bottom of the hollows in the inner surface IS and the summit of thepeaks.

Although it is not shown in the Figures, the dimension corresponding tothe nominal inside diameter of the tube can be denoted TED, as it isusually known in the art, i.e. here, the nominal inside diameter of thesmooth end sections ES1 and ES2.

According to a general aspect of the invention, the tube element TE has,on the twisted section CS, a nominal outside diameter COD comprisedbetween 18 and 30 millimetres. The pitch CP of the twist is less than3.5 millimetres. The depth CD is such that the ratio of the pitch CPraised to a real power R comprised between 1.5 and 2.5 to the depth CD,which is known as the form ratio FR, remains less than a threshold valueTV. In particular, the threshold value TV is close to 24. In particular,the power R is close to 1.7. In other words, the depth CD verifies theconditions COND1, COND2 and COND3 set out below:

FR=CP̂R/CD  (COND1)

FR<24  (COND2)

1.5<R<2.5, and in particular R=1.7  (COND3)

Tables 1A and 1B below show data characteristic of the twisted sectionCS for tube elements TE according to three variants of the invention(Table 1A), and two embodiments (Table 1B). The dimensions are hereexpressed in millimetres. In each case, the tube elements according tothese variants are such that they verify the conditions COND1, COND2 andCOND3, in particular where R=1.7.

TABLE 1A Variant 1 Variant 2 Variant 3 TOD 19.05 22.22 25.40 COD 18.9022.07 25.25 TT 0.5 0.5 0.5 CP between 2 and 3.5 between 2 and 3.5between 2 and 3.5 CD between 0.15 and between 0.15 and between 0.2 and0.3 0.3 0.3

TABLE 1B Embodiment 1 Embodiment 2 TOD 19.05 22.22 COD 18.90 22.07 TT0.5 0.5 CP 2 2.5 CD 0.16 0.25

Tables 1A and 1B show that the twisted part of the tubes according tothe invention has a very small pitch, less than 3.5 millimetres, andpreferably less than 3 millimetres, compared with the pitch valuesconventionally used in corrugated tubes, which are typically greaterthan 20 millimetres. The tubes according to the invention thereforediffer from conventional tubes in that the twisted section has a shapethat resembles a spiral.

Table 2 below shows dimensional characteristics relating to a set oftube elements (marked I, . . . , XII) each with a twisted centralsection. The tube elements differ from each other in the profile of therespective twisted section thereof, characterised by pitch CP and depthCD values that differ from each other. Dimensions absent from Table 2are common to the tubes I to XIV. In particular, the tube elements allhave an outside diameter of 22.22 millimetres and a wall thickness of0.5 millimetre. The tubes are made from grade 2 titanium.

The pitch CP and depth CD values are expressed in millimetres.

Table 2 also shows the corresponding values of the form ratio FR,calculated for a power R value of 1.7.

TABLE 2 CP CD FR I 2.0 0.14 23.21 II 2.3 0.19 21.92 III 2.5 0.25 18.84IV 3.1 0.17 39.44 V 3.2 0.39 18.06 VI 3.3 0.24 31.83 VII 3.3 0.32 24.16VIII 3.3 0.44 17.57 IX 4.2 0.47 24.42 X 4.3 0.49 23.96 XI 4.8 0.40 36.47XII 4.9 0.64 23.24 XIII 1.3 0.04 39 XIV 1.5 0.06 33

In Table 2, tube elements II and III are according to variant 2. Tubeelement III is also according to embodiment 2. In addition, elements I,II, III, V and VIII are according to the invention in that they havepitch values CP of less than 3.5 millimetres and also verify conditionsCOND1, COND2 and COND3.

Tube elements IV and VI have dimensions according to variant 2, with theexception that they do not verify conditions COND1 to COND3.

Tube element X verifies conditions COND1, COND2 and COND3 where R=1.7.

Table 3 below shows the results of heat exchange capacity measurementstaken on the elements in Table 2.

In Table 3, the coefficient K represents a heat exchange capacitymeasured for the tube element in question. The coefficient K isexpressed in Watt per square metre Kelvin (W·m⁻²·K⁻¹). The HER value,expressed as a percentage, corresponds to the improvement in the valueof K for the element in question compared with a smooth element withotherwise similar dimensions.

TABLE 3 K HER I 7,901 50% II 7,646 45% III 8,098 54% IV 6,669 26% V8,599 63% VI 6,771 28% VII 6,778 29% VIII 7,912 50% IX 6,659 26% X 8,08153% XI 6,553 24% XII 7,698 46% XIII 6473 23% XIV 6007 13%

Table 3 shows that compliance with conditions COND1, COND2 and COND3 isgenerally associated with a significant increase in heat exchangeperformance. The rows corresponding to tube elements I, II, III, V,VIII, X and XII have coefficient K values at least 45% greater than thereference value for a smooth tube (5,272 W·m⁻²·K⁻¹). A comparison inTable 2 of rows VII and X on one hand, and rows I, XIII and XIV on theother hand also shows a slight increase in heat exchange performancewhen the ratio FR exceeds the threshold value of 24. When the ratio FRis greater than the threshold value, the increase in heat exchangeperformance compared with a smooth tube is generally less than 30%.

Table 3 proves that the tube elements according to the invention havegreatly improved heat transfer capabilities compared with smoothelements on the one hand, and elements the twisted section of whichdeparts from the profile provided for by the invention.

Table 4 below shows the results of measurements taken on the tubeelements in Table 2.

Table 4 also shows the values of the coefficient known as the Darcy orDarcy-Weisbach coefficient, for the tubes in question, as well as theincrease DCR in the value of this coefficient compared with a smoothreference tube. The Darcy coefficient corresponds to a head losscoefficient. This dimensionless quantity represents the influence of thetype of flow (laminar or turbulent) and the finish of a pipe (smooth orrough) on the head loss. Here, the Darcy coefficient is calculated for aflow rate of 2.5 cubic metres per hour.

An increase in the Darcy value is overall unfavourable to theperformance of a tube element within a condenser. In particular, anincrease in the Darcy value implies an increase in the energyconsumption required for the circulation of the fluid inside the tubes.In other words, the increase in the Darcy value is detrimental to thecondensation of the steam on the outside of the tube element for thesame energy consumption.

TABLE 4 DARCY DCR I 0.032487593 48% II 0.032098063 46% III 0.03282328649% IV 0.052203857 129% V 0.054736888 140% VI 0.043978584 95% VII0.038960463 74% VIII 0.050991452 124% IX 0.071932623 211% X 0.068924461199% XI 0.052814985 132% XII 0.073328679 217% XIII 0.029523637 41% XIV0.03016296 44%

Table 4 shows in general terms that the tubes according to the inventionhave a significant increase in Darcy value. However, this increaseremains limited (less than 140 and lower than for certain tubes notaccording to the invention, as shown by a comparison with rows X andXII). Furthermore, the relative increase in the Darcy coefficient isvery small (close to, or even less than 100%) for the elements accordingto the second variant (tubes II and III) and for tube I, compared withthe other tubes tested. The tubes of the second variant and tube I havean increase in the Darcy value that is markedly less than for theothers.

As a result of the foregoing, the tubes with a twisted section accordingto the invention are capable of greatly improved performance regardingtheir capacity to condense a gas circulating outside the tube. Thisimproved performance results from a twisted shape that greatly improvesthe heat exchange capabilities and substantially limits the head losseffects.

The comparison between examples I and II with examples XIII and XIVshows that the lowering of the pitch allows to lower the Darcycoefficient, but fulfilling the conditions COND1, COND2, COND3 allows toimprove the heat exchange performance.

Furthermore, the tubes according to variants 1 to 3 and embodiments 1and 2 are capable of showing condensation performance that is furtherimproved due to heat exchange capabilities comparable to the other tubesaccording to the invention and substantially lower head losses comparedwith these tubes.

On the basis of the general embodiment of the invention and based ontests the results of which are at least partly given in Tables 2 to 3,it is considered that the features below, which are optional, additionalor alternative, are capable of further improving the condensationperformance of a tube:

The outside diameter of the tube TOD is between 19 and 26 millimetres,preferably between 20 and 26 millimetres, and even more preferentiallybetween 20 and 23 millimetres. In particular, the outside diameter TODis close to 19.05 millimetres, 22.22 millimetres or 25.4 millimetres.

The outside diameter COD of the twisted part is between 18 and 26millimetres, preferably between 20 and 26 millimetres, and even morepreferentially between 20 and 23 millimetres. In particular, the outsidediameter COD is close to 18.90 millimetres, 22.07 millimetres or 25.25millimetres.

The pitch CP is strictly greater than 2 millimetres. It is less than 3millimetres.

The depth CD is comprised between 0.05 and 0.6 millimetres, inparticular greater than 0.15 millimetres.

The thickness TT of the wall CW of the tube is between 0.4 and 1millimetre, for example of the order of 0.5 millimetre.

The invention is not limited to the embodiments described above, butencompasses all of the variants that a person skilled in the art mightimagine.

1-9. (canceled)
 10. A heat exchanger element comprising: a tubular bodywith a wall at least partly delimited by an inner surface and an outersurface, the wall having a twisted shape on at least one segment of thebody, and the inner surface including at least one groove with a shapecorresponding to the wall and extending helically over the segment,wherein on the segment, the outer surface has a diameter between 18 and30 millimeters, while the groove has a pitch of less than 3.5millimeters and a depth such that the ratio of the pitch at a real powerbetween 1.5 and 2.5 to the depth is less than a threshold value close to24.
 11. An element according to claim 10, wherein the real power isclose to 1.7.
 12. An element according to claim 10, wherein the bodyhas, on at least the segment, an outside diameter between 18 and 26millimeters.
 13. An element according to claim 10, wherein the body has,on at least the segment, an outside diameter between 20 and 26millimeters.
 14. An element according to claim 10, wherein the body has,on at least the segment, an outside diameter between 20 and 23millimeters.
 15. An element according to claim 12, wherein the body has,on at least the segment, an outside diameter close to 18.90, 22.07, or25.25 millimeters.
 16. An element according to claim 10, wherein thepitch is strictly greater than 2 millimeters.
 17. An element accordingto claim 10, wherein the pitch is less than 3 millimeters.
 18. Anelement according to claim 10, wherein the depth is between 0.05millimeter and 0.6 millimeter.
 19. An element according to claim 10,wherein the depth is greater than 0.15 millimeter.
 20. An elementaccording to claim 10, wherein the wall has a thickness between 0.4 and1 millimeter on at least part of the segment.
 21. An element accordingto claim 10, wherein the wall has a thickness close to 0.5 millimeter onat least part of the segment.
 22. A heat exchanger comprising at leastone element according to claim 10.